Effector Mechanisms of Humoral Immunity Flashcards
What is the complement system?
Consists of serum and cell surface proteins that interact with one another and with other molecules of the immune system in a highly regulated manner to generate products that function to eliminate microbes.
How are normal host cells not attacked by the complement system?
Complement activation is inhibited by regulatory proteins that are present on normal host cells and absent from microbes.
What are the three pathways of complement activation?
- classical
- alternative
- lectin
What are the two proteolytic complexes that complement activation depends on?
- C3 convertase
- C5 convertase
How is the alternative pathway activated?
C3b binding to various activating surfaces.
How is the classical pathway activated?
C1 binding to Ag-Ab complexes.
How is the lectin pathway activated?
Binding of a plasma lectin to microbes.
What does the spontaneous hydrolysis of plasma C3 lead to?
formation of a fluid-phase C3 convertase and the generation of C3b.
What happens if C3b is deposited on the surface of a microbe?
It binds factor B and forms the alternative pathway C3 convertase.
What makes up C5 convertase?
C3bBbC3b = C5 convertase
In the alternative pathway, what does properdin do?
Properdin stabilizes C3 convertases (C3bBb) on microbial surfaces.
In the alternative pathway, what does factor D do?
Plasma serine protease cleaves factor B when it is bound to C3b.
In the alternative pathway, what does factor B do?
Bb is a serine protease and the active enzymes of the C3 and C5 convertases.
In the alternative pathway, what does C3 do?
C3b binds to the surface of microbes and functions as an opsonin - also is a component of C3 and C5 convertases.
What is the classical pathway initiated by?
Binding of the C1 to IgG or IgM molecules that are bound to Ag.
Which IgG subclasses are the most efficient activators of complement?
- IgG3
- IgG1
What is C1 composed of in the classical pathway?
- C1q
- C1r
- C1s
What does C1q do?
Binds to Ab.
What does C1r and C1s do?
proteases
- C1r and C1s form a tetramer composed of two C1r and two C1s molecules*
- The ends of C1r and C1s contain the catalytic domains of these proteins*
What is the structure of C1q?
consists of 6 identical subunits arranged as radial arms.
How many portions of the Fc must C1 bind to to initiate the classical complement cascade?
C1 must bind to 2 or more Fc portions to initiate the complement cascade.
Are the Fc portions of soluble pentameric IgM Abs accessible to C1?
No
after IgM binds to surface-bound Ags, it undergoes a shape change that permits C1 binding and activation
What happens to IgM after it binds to surface-bound Ags so that C1 is able to bind to it?
After IgM binds to surface-bound Ags, it undergoes a shape change that permits C1 binding and activation.
How do soluble IgG molecules bind and activate C1 since they only have one Fc region?
Remember: C1 must bind to 2 or more Fc portions to initiate the classical complement cascade
Solbule IgG molecules will also not activate C1 because each IgG has only one Fc region, but after binding to cell surface Ags, adjacent IgG Fc portions can bind and activate C1.
How does the classical pathway form C3 convertase?
1) activated C1s cleaves the next protein in the cascade, C4, to generate C4b.
> C4b contains an internal thioester bond that forms covalent bonding with microbe to which the Ab is bound.
> It ensures that classical pathway activation proceeds on a microbial surface.
2) activated C1s then cleaves C2 to generate s soluble C2b fragment of unknown importance and a larger C2a fragment that remains physically associated with C4b.
3) the resulting C4b2a complex is the classical pathway C3 convertase.
**cleavage of C3 results in the removal of the small C3a fragment, and C3b can form covalent bonds with cell surfaces. Once C3b is deposited, it can bind factor B and generate more C3 convertase by the alternative pathway.
How is C5 convertase formed in the classical pathway?
> some of the C3b molecules generated by the classical pathway C3 convertase bind to the C3 convertase.
> The formed C4b2a3b complex functions as the classical pathway C5 convertase.
> It cleaves C5 and initiates the late steps of complement activation.
What does C1 do in the classical pathway?`
initiates the classical pathway
What does C1q do in the classical pathway?
binds to the Fc portion of Ab that has bound to:
- Ag
- apoptotic cells
- cationic surfaces
What does C1r do in the classical pathway?
serine protease, cleaves C1s to make it an active protease
What does C1s do in the classical pathway?
serine protase, cleaves C4 and C2
What does C4 do in the classical pathway?
> C4b covalently binds to a microbe and complement is activated.
> C4b binds C2 for cleavage by C1s.
> C4a stimulates inflammation (anaphylatoxin)
What does C2 do in the classical pathway?
C2a is a serine protease and functions as the active enzyme of C3 and C5 convertases to cleave C3 and C5.
What protein functions as the active enzyme of C3 and C5 convertases to cleave C3 and C5 in the classical pathway?
C2a
How is the lectin pathway activated?
Activation is triggered by the binding of microbial polysaccharides to circulating lectins, such as plasma mannose-binding lectin (MBL).
What does MBL bind to in the lectin pathway?
Mannose residues on bacterial polysaccharides.
What are the MBL-associated serine proteases that MBL associates with?
- MASP1
- MASP2
- MASP3
What are the MASP proteins in the lectin pathway?
Are structurally homologous to the C1r and C1s proteases and cleavage of C4 and C2 to activate the complement pathway.
True or False:
The subsequent events in the lectin pathway are identical to those that occur in the classical pathway.
True
In the lectin pathway, what does MBL do?
agglutinin, opsonin, complement fixing
In the lectin pathway, what does MASP1 do?
forms complex with MASP2 and collections or ficolins and activates MASP3
In the lectin pathway, what does MASP2 do?
forms complex with lectins, expecially ficolin-3
In the lectin pathway, what does MASP3 do?
associates with collectins or ficolins and MASP1 and cleaves C4
How is the MAC complex formed?
> C5 convertases cleave C5 into a small C5a that is released and a C5b fragment that remains bound to the complement proteins deposited on the cell surface.
> C5b binds C6 and C7
> C7 component of the resulting C5b, 6, 7 complex is hydrophobic, and it inserts into the lipid bilayer of cell membranes, where it becomes a high-affinity receptor for the C8 and C5b, 6, 7, 8 complex is formed.
True of False:
The formation of a fully active MAC is accomplished by the binding of C9 that polymerizes at the site of the bound C5b-8 to form pores in plasma membranes.
True
What does the C5 protein do?
> C5b initiates assembly of the MAC
> C5a stimulates inflammation
What does the C6 protein do?
component of the MAC: binds to C5b and accepts C7
What does the C7 protein do?
component of the MAC: binds to C5b, 6 and inserts into lipid membranes
What does the C8 protein do?
component of the MAC: binds to C5b, 6, 7 and initiates the polymerization of C9
What does the C9 protein do?
component of the MAC, polymerizes to form membrane pores
What is CR1 a high-affinity receptor for?
C3b and C4b
What does CR1 promote?
Phagocytosis of C3b- and C4b-coated particles and clearance of immune complexes from the circulation.
What do phagocytes use the CR1 receptor for?
To bind and internalize microbes and debris.
True of False:
CR1 also transduces signals that activate the killing mechanisms of the phagocytes.
True
What does CR2 bind?
Cleavage products of C3b (C3d, C3dg, and iC3b)
What does CR2 on B cells enhance?
CR2 enhance the responses of B cells to Ag.
What do CR2 on follicular DC do?
CR2 traps iC3-coated Ag-Ab complexes in the germinal centers.
Via which complement receptor does Epstein-Barr virus enter B cells?
CR2 - infects these cells, and can remain latent in infected cells for life.
What is CR3 (Mac-1)?
An integrin that functions as a receptor for the iC3b fragment generated by proteolysis of C3b.
What is CR3 involved in other than acting as a receptor for iC3b?
In the recruitment of leukocyte to sites of infection and tissue injury by binding to ICAM-1 on endothelial cells.
What does CR4 bind?
Binds iC3b, and the function of this receptor is probably similar to that of CR3.
List the complement regulator proteins?
> C1 inhibitor > factor I > factor H > membrane cofactor protein (MCP) > decay-accelerating factor (DAF)
What is the function of C1 INH?
displaces C1r2s2 from C1q and terminates classical activation
What is the function of decay-accelerating factor (DAF)?
classical C4b2a or alternative C3 convertase is dissociated by DAF
MCP and CR1, function similarly to DAF
In the presence of cell membrane-bound MCP or CR1 cofactors, what happens?
Plasma factor I proteolytically cleaves C3b attached to cell surfaces.
What does the presence of cell membrane-bound MCP or CR1 cofactors generate?
generates iC3b, and inactive form of C3b
What can factor H serve as a cofactor for?
factor I - mediated cleavage of C3b`
Which proteins inhibit formation of the MAC?
> membrane protein CD59 -> inhibits poly-C9 assembly
> S protein -> inhibits membrane insertion of C5b-C7
What are the three different complement functions?
> opsonization and phagocytosis
stimulation of inflammatory reactions
complement-mediated cytolysis
What happens to microbes on which complement is activated by the alternative or classical pathway (most sensitive)?
Microbes become coated with C3b, iC3b, or C4b and are phagocytized by the binding of these proteins to specific receptors on macrophages and neutrophils.
Which proteolytic complement fragments induce acute inflammation by activating mast cells, neutrophils, and endothelial cells?
C5a, C4a, and C3a
What happens by binding to antigen-antibody complexes?
complement promotes solubilization of these complexes and their clearance by phagocytes
True of False:
C3d protein generated from C3 binds to CR2 on B cells and facilitates B cell activation and the initiation of humoral immune responses.
True
What is the most common human complement deficiency?
C2 deficiency
Deficiencies in which complement fragments is associated with systemic lupus erythematosus?
C1q, C2, and C4 deficiencies
True of False:
C2 and C4 deficiencies are not usually associated with increased susceptibility to infections because the alternative pathway may compensate the deficiency.
True
What is C3 deficiency associated with?
Frequent serious pyogenic bacterial infections that may be fatal.
What do deficiencies in properdin and factor D result in?
Susceptibility to infection with pyogenic bacteria.
What do deficiencies in C5-C9 result in?
Disseminated infections by Neisseria bacteria.
True or False:
Complement system cay cause significant tissue damage.
True
True of False:
Some of the pathologic effects associated with bacterial infections may be due to complement-mediated acute inflammatory responses to infectious organisms.
True
What is complement activation associated with?
Intravascular thrombosis and can lead to ischemic injury to tissues.
What are the pathologic effects in an autoantibody-mediated kidney disorder?
Membranous nephropathy damage to glomerular epithelial cells can be mediated by the MAC that is generated after Ab binds to a glomerular auto-Ag.
How can microbes evade the complement system?
By recruiting host complement regulatory proteins.
Many pathogens express sialic acids. How do they use this to evade the complement system?
Sialic acids inhibit the alternative pathway of complement by recruiting Factor H, which displaces C3b from Bb.
many other pathogens have evolved proteins that facilitate the recruitment of factor H to their cell walls.
How does HIV evade the host’s complement system?
Incorporates multiple host regulatory proteins into their envelopes.
For instance, HIV incorporates the GPI-anchored complement regulatory proteins DAF and CD59 when it buds from an infected cell.
True of False:
Neonates lack the ability to mount effective immune responses against microbes, and for several months after birth, their major defense against infection is passive immunity provided by maternal antibodies.
True
What is the transport of maternal IgG across the placenta and across the neonatal intestinal epithelium mediated by?
IgG-specific Fc receptor called the: neonatal Fc receptor (FcRn).
How are newborns protected from infection?
By maternal Abs transported across the placenta into the fetal circulation.
ingested IgA and IgG can neutralize pathogenic organisms that attempt to colonize the infant’s gut
Which maternal Ab is ingested by the nursing infant through breast milk?
maternal IgA
By what process is maternal IgA from the mother’s milk transported across the gut epithelium of newborns?
transcytosis
What are the effector functions of Abs?
1) neutralize microbe toxins
2) opsonize them for phagocytosis
3) sensitize them for Ab-dependent cellular cytotoxicity
4) activate the complement system
What is the protective mechanism in vaccine-induced humoral immunity for Polio?
Neutralization of virus by mucosal IgA Ab.
What is the protective mechanism in vaccine-induced humoral immunity for tetanus, diphtheria?
Neutralization of toxin by systemic IgG Ab
What is the protective mechanism in vaccine-induced humoral immunity for Hepatitis A or B?
Neutralization of virus by mucosal IgA or systemic IgG Ab.
What is the protective mechanism in vaccine-induced humoral immunity for Pneumococcal pneumonia, Haemophilus?
Opsonization and phagocytosis mediated by IgM and IgG Abs, directly or secondary to complement activation.
What does the influenza virus have on its envelope to infect respiratory epithelial cells?
Hemagglutinin
What do Gram-negative bacteria use to attach to and infect a variety of host cells?
Pili
How do Abs neutralize microbes and toxins released from microbes?
Abs BIND to these microbial structures and interfere with the ability of the microbes to interact with cellular receptors by means of steric hindrance and may thus, prevent infection.
True of False:
Antibodies inhibit the spread of microbes from an infected cell to an adjacent uninfected cell.
True
True or False:
Abs block the binding of toxins to cells and thus inhibit the pathologic effects of the toxins.
True
Which isotype of antibodies is involved in Ab-mediated opsonization and phagocytosis?
IgG - which opsonize microbes and promote their phagocytosis by binding to Fc receptors on phagocytes.
What are the 4 types of Fc receptors?
> Fc-gamma-RI (CD64)
Fc-gamma-RII (CD32)
Fc-gamma-RIII (CD16)
Fc-epsilon-RI
Which cell types have Fc-gamma-RI (CD64) receptors?
- macrophages
- neutrophils
- eosinophils
Which cell types have Fc-gamma-RII (CD32) receptors?
- macrophages
- neutrophils
- dendritic cells
- B cells
- NK cells
Which cell types have Fc-gamma-RIII (CD16) receptors?
NK cells
Which cell types have Fc-epsilon-RI receptors?
- mast cells
- basophils
- eosinophils
What is the function of Fc-gamma-RI (CD64) receptors?
- phagocytosis
- cell activation
What is the function of Fc-gamma-RII (CD32) receptors?
- phagocytosis
- cell activation
- feedback inhibition
What is the function of Fc-gamma-RIII (CD16) receptors?
- Ab-dependent cell-mediated cytotoxicity
What is the function of Fc-epsilon-RI receptors?
- cell activation (degranulation)
Which two isotypes are the most efficient opsonins for promoting phagocytosis via high-affinity Fc-gamma-RI (CD64)?
IgG1 and IgG3
How do Ag-Ab complexes block B cell activation?
Hint: Fc-gamma-RIIB receptor.
> Ag-Ab complexes simultaneously bind to the BCR and the Fc-gamma-RIIB receptor through the Fc portion of the Ab.
> As a consequence of this simultaneous ligation of receptors, phosphatases (which is SHIP - converts PIP3 to PIP2) associated with the cytoplasmic tail of the Fc-gamma-RIIB inhibit signaling by the BCR complex and block B cell activation.
Explain FcR-mediated Ab feedback?
> It is triggered by ecreted Ab and blocks further Ab production as apposing the activation via CR2.
> It is a physiologic control mechanism in humoral immune responses.
A likely scenerio is that IgM which activate complement but do not bind to the Fc-gamma-R are involved in amplification, whereas increasing production of IgG leads to feedback inhibition
Regulatory functions of immune complexes.
> immune complexes bind to activating FcRs and inhibitory FcRs that are expressed by immune effector cells.
> on plasma cells, which produce high levels of Ag-specific Abs, BCR expression is very low or absent, resulting in exclusive triggering of inhibitory signaling pathways.
What is Ab-dependent cell-mediated cytotoxicity (ADCC)?
> Ab of certain IgG subclasses bind to infected host cells, and the Fc regions of the bound Ab are recognized by an Fc-gamma-RIII on NK cells.
> The NK cells are activated and kill Ab-coated cells.
True or False:
Worms are too large to be engulfed by phagocytes and they are relatively resistant to the microbicidal products of neutrophils and macrophages.
True
What Ab isotype and cells function together to mediate the killing and expulsion of some helminthic parasites?
- IgE
- eosinophils
- mast cells
What protein present in the granules of eosinophils kill worms (helminths)?
Worms can be killed by a toxic cationic protein, known as the MAJOR BASIC PROTEIN, present in the granules of eosinophils.
